1. Field of the Invention
The present invention relates to a polypropylene-based resin composition excellent in dimensional stability and molded article thereof, more specifically a polypropylene-based resin composition excellent in dimensional stability and, at the same time, well-balanced in linear expansion coefficient, fluidity, rigidity, impact resistance and lightness, and molded article thereof.
The present invention also relates to a polypropylene-based resin composition excellent in recyclability, more specifically a polypropylene-based resin composition well-balanced in mechanical properties (e.g., rigidity, heat resistance and impact resistance) and excellent in lightness, and molded article thereof.
2. Description of the Prior Art
Polypropylene is a thermoplastic resin easily molded, and excellent in rigidity, heat resistance, impact resistance and recyclability. As such, it has found wide uses in automobile parts, e.g., bumpers, instrument panels and door trims. More recently, its applications are expanding into areas where metals, e.g., iron, have been predominant.
Polypropylene is higher in shape freedom but lower in dimensional stability than metallic materials. In particular, it has been finding outdoor uses subject to large temperature fluctuations, e.g., automobile external plates, and causes problems, e.g., deteriorated quality at gaps and waving, to damage vehicle outer appearances.
Therefore, it has been incorporated with an inorganic filler, e.g., talc, calcium carbonate or mica, or elastomer component, in order to improve its dimensional stability, described above, and moldability, mechanical properties and outer appearances, among others. These attempts, however, have failed to produce sufficient effects.
Moreover, incorporation of low-viscosity rubber has been proposed to facilitate orientation of the rubber component in the polypropylene matrix (JP-A 2002-249635). This effect, however, is still insufficient.
Still more, methods have been proposed to further improve dimensional stability of polypropylene by combining talc with whiskers of potassium titanate or the like (JP 2,521,380, 2,566,469 and 2,622,891). However, their products are still insufficient in dimensional stability when they are to be used as substitutes for metals, because linear expansion coefficient which they can attain is limited to 3 to 4×10−5 cm/cm° C. or so. Therefore, polypropylene has been still demanded to be further improved in dimensional stability.
These related art techniques intentionally incorporate a low-viscosity rubber component to control linear expansion coefficient. A low-viscosity rubber, although effectively controlling linear expansion coefficient, deteriorates performance with respect to impact resistance. Therefore, development of polypropylene of well-balanced impact resistance and dimensional stability has been demanded.
Demands for polypropylene have been steadily growing worldwide, which has brought major challenges of recycling for reutilization/reuse of spent polypropylene products. For the products which are required to have a high rigidity, e.g., automobile bumpers, polypropylene is incorporated with an inorganic filler, represented by talc, to realize a target rigidity (JP 3,031,986, and JP-A 6-57054, 2002-20560 and 2002-3691).
Compositing polypropylene with an inorganic filler invariably increases its density, which counters trends towards reducing weight. At the same time, ashes are produced when the spent products are incinerated, which impedes recycling of polypropylene for heat sources. Techniques for compositing polypropylene with carbon fibers are known. However, carbon fibers are not well dispersed in the polypropylene resin matrix. The general countermeasures to improve the dispersion include incorporation of a modified resin, e.g., modified polypropylene, and pretreatment of carbon fibers with a special resin for surface modification or the like (JP-A 8-48816, 6-41389 and 2001-294760).
Use of a modified resin for the above purpose tends to deteriorate excellent properties a polypropylene resin inherently has, e.g., rigidity and impact resistance, and is not recommended for products which are required to have high rigidity and impact resistance, e.g., those for automobiles. On the other hand, carbon fibers, when incorporated in the polypropylene matrix, are easily cut to become too short in the composite to improve rigidity. Therefore, carbon fibers are difficult to sufficiently exhibit their effects at a normal content.